The standard deviation is a bit troublesome, from the known1/20 (x12+x2 2+...+x202-20 * 902) = 36.

Arrange x12+x22+...+x202 =162720.

Similarly, x212+...+x402 =128320.

So the variance of the whole class is1/40 (x12+...+x402-40 * 852) =1/40 (162720+028320-289000).

Standard deviation =sqrt53 is about 7.

Explain that the number after x is subscript.